nice spectrogram! That modulated carrier on 17.6 kHz looks like local QRM to me - motor controller, dog fence, lawnmower guide wire, or anything like that.
The MSK traces on 23.4 and 22.8 are presumably NLK and NML, mirrored down at the 24 kHz Nyquist limit.
Apropos NML: Have you tried to zoom in on HWU 21.75 kHz? You might see similar fringes to those we've been seeing on NML and others every night:
With the help of Peter (G3PLX) in the UK and Edgar in Tasmania, we have obtained some data on differential delays and direction of arrival of the reflected component. I have come to believe that the signals may be bouncing off the equatorial electrojet near
the magnetic equator.
Von: Laurence KL7 L <[email protected]
An: rsgb_lf_group <[email protected]
Verschickt: Fr, 17. Feb 2017 21:24
Betreff: Re: VLF: Tacamo on 17.8 and 29.6 kHz